Particle classifier

ABSTRACT

A new installment classifies particles by their aerodynamic size This installment produces a truly monodisperse aerosol, and can produce very narrow distributions over a wide range of sizes Particles suspended in a fluid are classified by supplying particles into suspension in a carrier flow of a fluid and providing an acceleration to the flow at an angle to the velocity of the flow to cause the particles to follow trajectories determined by the acceleration and drag on the particles caused by the fluid The particles are then classified according to their trajectories The installment has a flow channel and a source of particles to supply particles into suspension in a carrier fluid in the flow channel A drive is connected to the flow channel at an angle to the flow of fluid through the carrier flow channel, and a classification system classifies the suspended particles according to their trajectories

TECHNICAL FIELD

Aerosol particle classifiers.

BACKGROUND

Aerosol classifiers are used to produce a monodisperse aerosol, that is,they select a narrow range of particles from a larger distribution ofparticles. This method is used for many applications including;nano-particle generation, measuring distributions of particles in air,measuring the deposition of particles in filters and other devices,sampling ambient aerosols, and many others. These measurements are oftendone in research areas as diverse as: nano-technology, pharmaceuticalresearch, health-effects studies, inhalation toxicology, bio-aerosoldetection, filter testing, indoor-air quality studies, industrialhygiene, energy and combustion research, automotive emissionsmeasurements, and atmospheric and climate-change research.

Currently, the most commonly used classifier is called the DifferentialMobility Analyzer (DMA, Knutson and Whitby 1975). The DMA classifiesparticles based on their electrical mobility, that is, the motion of acharged particle in an electrostatic field. By controlling theelectrostatic field and the flow between two cylinders the particles areclassified by their electrical mobility, which is related to the numberof electric charges on the particle and the drag experienced by theparticle, which is a function of the particle's size and shape. Fornon-spherical particles an equivalent diameter, called the electricalmobility equivalent diameter is defined for these particles, which havethe same electrical mobility of a spherical particle of the same size.To classify particles with this instrument an electric charge must beplaced on these particles using charging methods such radioactive-sourcecharge neutralizers or corona discharge. However, with all chargingmethods not a single charge is placed on each particle but rather adistribution of charges are placed on the population of particles. Forexample, particles may obtain one, two, three, or more positive charges;one, two, three, or more negative charges or no charge at all. Theelectrical mobility of the particles is a function of the number ofcharges on the particle and its drag. Therefore, a smaller particle withone charge will have the same electrical mobility as a larger particlewith two charges. Thus, the aerosol sample that is classified by the DMAwill not be truly monodisperse in terms of particle size, but rather itwill have a mix of sizes corresponding to an integer number of chargedparticles. Techniques are used to minimize the number of charge statesbut the DMA can never produce a truly monodisperse aerosol. For someapplications (like measuring size distributions) the error introduced bythe charge distribution can be corrected using inversion techniques, butit can never be fully eliminated. In other applications and experiments,these extra particle sizes can degrade performance or skew results.

Another technique has been used to classify particles by theirmass-to-charge ratio is an instrument called the Aerosol Particle Massanalyzer (APM; Ehara et al. 1996; Ehara 1995) or the Couette CentrifugalParticle Mass Analyzer (Couette CPMA; Rushton and Reavell 2004; Olfertand Collings 2005). With these instruments charged particles areclassified between two rotating cylinders with electrostatic andcentrifugal forces. A similar charging mechanism is applied to chargethe particles. Therefore, particles of the same mass-to-charge ratiowill be classified. For example, a particle with one charge will beclassified at the same time as a particle with twice the mass and twicethe number of charges. Therefore, the APM or Couette CPMA do not producea truly monodisperse aerosol.

Other aerosol and particle instruments are based on measuring what iscalled the ‘aerodynamic’ diameter of the particle. The aerodynamicequivalent diameter is defined as the diameter of a spherical particlewith a density of water that has the same terminal velocity as theactual particle. Instruments that measure the aerodynamic size ofparticles include various kinds of impactors (Marple et al., 1991;Keskinen et al., 1992), virtual impactors (Conner, 1966), andaerodynamic lenses (Liu et al., 1995a, 1995b). However, these methodsonly provide a means of dividing the aerosol sample in half, whereparticles larger than the cut-off point are classified in one direction(i.e., impacted onto the impaction plate) and particles smaller than thecut-off point continue with the flow. Often, several of these stages arestacked together to provide classification into several large bins.There is currently no instrument that classifies particles by theiraerodynamic diameter and produces a monodisperse aerosol.

SUMMARY

The applicant has devised a new instrument, called the AerodynamicParticle Classifier (APC) that provides classification of particles. Inan embodiment, a method of classification of particles suspended in afluid is provided comprising the steps of providing a carrier flow of afluid, supplying particles into suspension in the carrier flow,providing an acceleration to the flow at an angle to the velocity of theflow to cause the particles to follow trajectories determined by theacceleration and drag on the particles caused by the fluid, andclassifying the particles according to the trajectories of theparticles. The particles may be classified for example by splitting aflow containing the particles or by detecting impacts of the particleson boundaries of a flow channel containing the flow.

The following are features any or all of which may be provided incombination with the above method of classification of particles: thefluid may be a gas such as air; the carrier fluid may be caused torotate around an axis by the rotation of one or more conveying flowchannels; the acceleration may be centripetal acceleration; the step ofsupplying particles into suspension in the carrier flow may comprisemerging a flow of a fluid containing suspended particles into thecarrier flow; the step of classifying particles according to thetrajectory of the particles may comprise splitting the carrier flow intotwo or more flows; and the step of classifying the particles accordingto the trajectory of the particles may comprise supplying a surface atwhich particles may impact depending on their trajectory.

Also provided is an apparatus for classifying particles suspended in afluid, the apparatus comprising: elements defining one or more carrierflow channels, a source of a carrier fluid flow into the carrier flowchannel, a source of particles connected to supply the particles intosuspension in the carrier fluid in the carrier flow channel, a driveconnected to operate on the elements defining the carrier flow channelto supply an acceleration to the elements defining the flow channel atan angle to the flow of fluid through the carrier flow channel, and aclassification system for classifying the suspended particles accordingto their trajectories.

The following are features all or any of which may be provided incombination with the above apparatus for classifying particles: thecarrier fluid may be a gas; the carrier flow may be caused to flowthrough one or more flow channels caused to rotate around an axis: theflow channels may be sectors or the whole of an annular space defined byinner and outer walls which are surfaces of revolution around an axisclose to the axis of rotation; the surfaces of revolution may besubstantially cylindrical in shape; the drive may comprise a motorconnected to cause rotation of the elements defining the carrier flowchannel; the source of particles connected to supply the particles intosuspension in the carrier fluid in the carrier flow channel or channelsmay comprise elements defining a suspension flow channel or channelswhich intersect the carrier flow channels, the suspension flow channelsbeing capable of directing a fluid containing suspended particles intothe carrier flow channels; the classification system may compriseelements defining a split of each of the carrier flow channels into twoor more channels; the classification system may comprise a surface ineach carrier flow channel at which particles suspended in fluid in thecarrier flow channel may impact depending on their trajectory; and thesurface may be an element defining or partially defining the carrierflow channel.

These and other aspects of the device and method are set out in theclaims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described with reference to the figures, inwhich like reference characters denote like elements, by way of example,and in which:

FIG. 1 is a schematic of an Aerosol Particle Classifier (APC) (not toscale) with a cylindrical flow path;

FIG. 2 is a diagram showing details of the particle trajectory and flowsbetween the cylinders in the embodiment of FIG. 1;

FIG. 3A is a graph of the normalized transfer function of the APC ofFIG. 1;

FIG. 3B is a graph of the transfer function of the APC of FIG. 1 interms of aerodynamic diameter for the operating conditions given in thedescription;

FIG. 4 is a schematic of an APC (not to scale) with a partial cylinderflow path;

FIG. 5 is a schematic of an APC (not to scale) with a curved flow pathwith boundaries shaped as surfaces of revolution;

FIG. 6 is a schematic of an APC (not to scale) with detectors on anouter cylinder defining the flow path;

FIG. 7A is a schematic showing an aerodynamic classifier with a particlecounter; and

FIG. 7B is a schematic showing a particle charger with an aerodynamicclassifier of the embodiment of FIG. 6.

DETAILED DESCRIPTION

FIGS. 1 and 2 show diagrams of an exemplary embodiment of the APC,generally denoted by 100. The APC disclosed here comprises elementsdefining a carrier flow channel, here two concentric cylinders, an innercylinder 102 and an outer cylinder 104 rotating in the same directionand at a similar rotational speed (normally the two cylinders would berotating at the same rotational speed although different speeds can alsobe used, see below). Other surfaces of revolution (axially symmetricshapes, synonymously surfaces of rotation) than cylinders may also beused. In an embodiment a flow channel may be defined by partialcylinders, e.g. sectors of a cylinder that do not extend in a fullcircle around the central axis of the cylinders, or partial surfaces ofrevolution, and by substantially radial surfaces between the inner andouter surfaces. If a flow channel is defined by partial cylinders orpartial surfaces of revolution then the surfaces could form a singleelement defining the flow channel. Referring to FIG. 1, in theembodiment shown the cylinders are attached to a rotating shaft 120mounted on bearings 122 and rotated via pulley 124. These elements actas a drive to operate on the elements defining a carrier flow channel(in this embodiment by rotating them) to supply an acceleration (here acentripetal acceleration) to the elements defining the flow channel atan angle to the flow of fluid through the carrier flow channel.Referring to FIG. 2, a source of particles is connected to supplyparticles into suspension in the carrier fluid, in this embodiment slit118 acting as a source. The particles, carried along by the aerosol flow106 with flow rate Q_(a), enter the gap between the two cylindersthrough slit 118 in the inner cylinder wall. A sheath flow 108 with flowrate Q_(sh) is also introduced from a source of carrier fluid flow intothe carrier flow channel between the two cylinders. In this embodimentan initial flow channel 126 acts as a source of carrier fluid byintroducing the sheath flow into the carrier flow channel. It is assumedthat flow is laminar and incompressible, which is a reasonableassumption for the geometry, flow rates, and gas pressure used in normaloperation. In this embodiment the flow is axial in the frame ofreference of the rotating cylinders, and tangential to the cylinders orto an imaginary cylinder coaxial with the cylinders; in an embodimentwith cylinders rotating at different speeds the flow may still betangential. In the absence of any centrifugal force (due to centripetalacceleration of the fluid containing the particles) the particles wouldtravel between the two cylinders between the inner cylinder wall and theaerosol streamline 110. However, when the cylinders are rotated, theparticles experience a centrifugal force in the direction of the outercylinder and a drag force toward the centre of rotation. The centrifugalforce both supplies the particles into the carrier flow and imparts acomponent of velocity across the carrier flow. Thus, in this example,the particles are not pre-mixed. The particles will also travel in theaxial direction carried along by the aerosol flow and sheath flow.Therefore, the velocity of particles in the radial (v_(r)) and axial(v_(z)) direction will be:

$\begin{matrix}{{v_{r} = {\frac{r}{t} = {{\frac{C_{c}}{3{\pi\mu}\; d_{p}}m\; \omega^{2}r} = {{\tau\omega}^{2}r}}}}{and}{{v_{z} = {\frac{z}{t} = u_{z}}},}} & (1)\end{matrix}$

where r is the radial position of the particle, ω is the rotationalspeed of the cylinders, m is the mass of the particle, d_(p) is thediameter of the particle, μis the viscosity of the carrier gas, C_(c) isthe Cunningham slip correction factor for the particle, and u_(z) is thevelocity of the carrier gas in the axial direction. It will be assumedthat the velocity profile is uniform (i.e., u_(Z) is constant). Theparticle relaxation time, τ, is defined as,

$\begin{matrix}{{\tau = {{\frac{C_{c}}{3\pi \; \mu \; d_{p}}m} = {\frac{C_{c}\rho_{p}d_{p}^{2}}{18\mu} = \frac{C_{c}\rho_{0}d_{ae}^{2}}{18\mu}}}},} & (2)\end{matrix}$

where ρ_(p) is the true particle density, ρ₀ is unit density (1000kg/m³), and d_(ae) is the so-called aerodynamic diameter of theparticle.

Using the chain rule and differentiating, the radial position of theparticle can be found as a function of the axial position,

$\begin{matrix}{{{r(z)} = {r_{in}{\exp\left( \frac{{\tau\omega}^{2}z}{u_{z}} \right)}}},} & (3)\end{matrix}$

where r_(in) is the initial position of the particle when it enters theclassifier.

A classification system classifies the suspended particles according totheir trajectories. In the embodiment shown particles are classifiedaccording to whether their trajectories bring them through sampling exit114. The transfer function of the instrument (the distribution ofparticles that leave the classifier at any given operating condition)can be found by determining the trajectory of the particles. A sampleflow 112 with flow rate Q_(s) exits the classifier through sampling exit114. In the embodiment shown, the sample flow is part of the sheathflow. The remainder of the sheath flow and the aerosol flow exit theclassifier as exhaust flow 116. Defining r₁ as the outer radius of theinner cylinder, r₂ as the inner radius of the outer cylinder, r₃ as theouter radius of the aerosol flow, and r₄ as the inner radius of thesample flow, the largest particle (i.e., the largest τ) that will passthrough the classifier, exiting the classifier in the sample flow 112,will start at r_(in)=r₁ and will reach r₂ at the end of the classifier(z=L). Therefore,

$\begin{matrix}{\tau_{\max} = {{u_{z}\frac{\ln \left( {r_{2}/r_{1}} \right)}{\omega^{2}L}} = {\frac{\left( {Q_{sh} + Q_{a}} \right)}{\pi \left( {r_{2}^{2} - r_{1}^{2}} \right)}{\frac{\ln \left( {r_{2}/r_{1}} \right)}{\omega^{2}L}.}}}} & (4)\end{matrix}$

The smallest particle that will be classified, τ_(min), will enter theclassifier at r_(in)=r₃ and will reach r₄ at the end of the classifier.The radii r₃ and r₄ can be related to the radii r₁ and r₂, realizingthat for uniform flow,

$\begin{matrix}{u_{z} = {\frac{Q_{sh} + Q_{a}}{\pi \left( {r_{2}^{2} - r_{1}^{2}} \right)} = {\frac{Q_{sh}}{\pi \left( {r_{2}^{2} - r_{3}^{2}} \right)} = {\frac{Q_{s}}{\pi \left( {r_{2}^{2} - r_{4}^{2}} \right)}.}}}} & (5)\end{matrix}$

Therefore,

$\begin{matrix}{\tau_{\min} = {\frac{\left( {Q_{sh} + Q_{a}} \right)}{\pi \; \omega^{2}{L\left( {r_{2}^{2} - r_{1}^{2}} \right)}}{{\ln\left( \frac{Q_{sh} + Q_{a} + {Q_{s}\left( {1 - {r_{1}^{2}/r_{2}^{2}}} \right)}}{Q_{sh} + Q_{a} - {Q_{sh}\left( {1 - {r_{1}^{2}/r_{2}^{2}}} \right)}} \right)}.}}} & (6)\end{matrix}$

Particles with τ>τ_(max) will intercept the outer cylinder wall beforereaching the exit slit and will adhere to the cylinder surface, whileparticles with τ<τ_(min) will flow past the exit slit and be carried outof the instrument with the exhaust flow. The particles adhere to thewall of the outer cylinder due to van der Waals forces (Friedlander,2000) and will remain there until the cylinder is cleaned. (Like theDMA, under normal operating conditions and aerosol concentrations, thecylinder will only need to be cleaned once every few months.) Betweenthe maximum and minimum relaxation times, only a fraction of theparticles will be classified. A particle must migrate into the sampleflow, defined by the sample streamline (r₄≦r<r₂), by the time theparticle has reached the end of the classifier. For particles withτ>τ_(min), only particles with an initial radial position r_(c)≦r<r₃will be classified, where r_(c) is called the critical radius. Thelimiting trajectory for τ>τ_(min) will be the particle that starts atr_(c) and reaches r₄. Substituting this condition into Eq. 3 and solvingfor the aerosol fraction, f₁, that is classified reveals,

$\begin{matrix}{f_{1} = {\frac{\begin{matrix}{Q_{sh} + Q_{a} - {\exp\left( \frac{{- 2}{\tau\omega}^{2}L\; {\pi \left( {r_{2}^{2} - r_{1}^{2}} \right)}}{Q_{sh} + Q_{a}} \right)}} \\\left( {Q_{sh} + Q_{a} - {Q_{s}\left( {1 - {r_{1}^{2}/r_{2}^{2}}} \right)}} \right)\end{matrix}}{Q_{a}\left( {1 - {r_{1}^{2}/r_{2}^{2}}} \right)} - {\frac{Q_{sh}}{Q_{a}}.}}} & (7)\end{matrix}$

Likewise, for particles with τ<τ_(max), the particles starting at thecritical radius, r_(c), must reach r₂ by the end of the classifier. Inthis case the fraction of the aerosol, f₂, that is classified is,

$\begin{matrix}{f_{2} = {\frac{Q_{sh} + Q_{a}}{Q_{a}}{\left( {1 - \frac{1 - {\exp\left( \frac{{- 2}{\tau\omega}^{2}L\; {\pi \left( {r_{2}^{2} - r_{1}^{2}} \right)}}{Q_{sh} + Q_{a}} \right)}}{1 - {r_{1}^{2}/r_{2}^{2}}}} \right).}}} & (8)\end{matrix}$

Furthermore, if the sample flow rate is smaller than the aerosol flowrate, then the transfer function cannot be larger than, f₃=Q_(s)/Q_(a).

The transfer function, Ω, will be the minimum of these three fractionsor one. Therefore, the transfer function can be expressed as,Ω=max[0,min(f₁,f₂,f₃,1)].

The normalized transfer function is shown in FIG. 3A, where thenormalized particle relaxation time is defined as τ/τ*. The value τ* isthe particle relaxation time at the centre of the transfer function andis defined as τ*=(τ_(max)+τ_(min))/2. The half-width of the transferfunction is defined as, Δτ=(τ_(max)−τ_(min))/2.

It can be shown, that when the gap between the cylinders is small andQ_(a)=Q_(s), then the relative width of the transfer function, Δτ/τ*, isJust the ratio of the aerosol to sheath flow rates, Δτ/τ*=Q_(a)/Q_(sh).To produce a highly monodisperse aerosol this ratio should approximatelybe in the range of 0.05 to 0.1.

As an example some sample dimensions and operating conditions that arewell suited for most applications are: r₂=37 mm, r₁=35 mm, L=200 mm,Q_(sh)=3 L/min, and Q_(a)=Q_(s)=0.3 L/min. In general, it is beneficialto keep the gap between the cylinders relatively small (i.e.(r₂−r₁)<<r₁), as smaller gaps increase the height of the transferfunction. During operation the flow rates maybe changed to vary thewidth of the transfer function as desired as long as the flow remainslaminar. An example of a transfer function of the APC is shown in FIG.3B using the given dimensions. In this example, when the rotationalspeed is 5650 rpm the centre of the non-diffusion transfer function willbe at d_(ae)*=100 nm, with the minimum and maximum sizes classified at92 nm and 108 nm, respectively. A more complicated model was alsodeveloped which accounts for the effects due to particle diffusionwithin the classification region. The diffusion transfer function isalso shown in FIG. 3 b and it shows that the transfer function becomesbroader and shorter when diffusion is included. This affect will becomemore prevalent for smaller particle sizes.

Thus, this embodiment of the APC will produce a very narrow, ormonodisperse, size distribution. Using these same dimensions, the APCwould be able to classify particles over an extremely wide range forexample 10 nm to 10 μm using rotational speeds ranging from 20,000 to 95rpm and smaller particle sizes could be classified by using higherrotational speeds (by comparison the DMA is typically used over a rangeof approximately 2.5 to 1,000 nm).

It should be noted that the analysis used here is very similar to theproven theoretical analysis used in the DMA, with the exception that theAPC has a centrifugal force instead of an electrostatic force toclassify the particles. Therefore, we have a sound basis for predictingthat this theoretical model of the APC will closely match experimentaldata once a prototype is developed.

The above description of the embodiment of the APC of FIG. 1 is just oneway the instrument could be designed or configured. Other alternativedesigns can be envisioned depending on the application or requirements.These include: 1) the use of rotating channels instead of a continuouscylindrical section, 2) two cylinders rotating a slightly differentrotational speeds, and 3) ways to measure aerosol size distributions.

The embodiment of FIG. 1 uses concentric cylinders to classify theparticles. However, other rotating geometries may also be used. Forexample, partial cylindrical sections such as sectors of a cylinder maybe used as elements defining one or more carrier flow channels or otherlong channels attached to a rotating shaft may be used as the carrierflow channels. However, the analysis of the transfer function willchange with these different geometries, where the cylindrical geometryis the simplest to analyze. Referring to FIG. 4, an APC 200 is shownwith a rotating flow channel (multiple flow channels may be includedaround the central axis, but only one is shown in the figure). The flowchannel may be defined by an inner partial cylindrical section 202 andan outer partial cylindrical section 204 although other shapes thanpartial cylindrical sections are possible. The embodiment may operate ina similar way as the embodiment of FIG. 1 except that the flow channeldoes not extend all the way around the central axis. In particularrotating shaft 220, bearings 222, pulley 224, sheath flow 208, aerosolflow 206, initial flow channel 226, sample flow 212 and exhaust flow 216may be similar to their counterparts of FIG. 1 and cooperate similarly,except that the portions of the sheath flow and sample flow within therotating parts of the classifier may not extend all the way around thecentral axis. Referring to FIG. 5, an APC 300 is shown with a flowchannel defined by an inner surface 302 and an outer surface 304 whichare not cylinders. The surfaces may be surfaces of revolution. However,it would also be possible to use different shapes including partialsurfaces of revolution that extend only part of the way around thecentral axis, as in FIG. 4. Further shapes other than partial surfacesof revolution may also be used. The embodiment shown in FIG. 5 mayoperate in a similar way as the embodiment of FIG. 1 except for thedifferent shape of the flow channel. In particular, rotating shaft 320,bearings 322, pulley 324, sheath flow 308, aerosol flow 306, initialflow channel 326, sample flow 312 and exhaust flow 316 may be similar totheir counterparts of FIG. 1 and cooperate similarly, except where adifferent shape is appropriate to accommodate the shape of the flowchannel.

In the analysis relating to FIGS. 1 and 2 it was assumed that thecylinders were rotating at the same rotational speed. However, cylindersrotating at slightly different speeds can also be used. In this case itwould be preferable to rotate the inner cylinder slightly faster thanthe outer cylinder. When this is done, and the speed difference is largeenough, the centrifugal force will decrease as its radial positionincreases. This causes the particle trajectories in the classifier toslightly ‘converge’ near the end of the classifier, resulting in ahigher transfer function. A similar method is used in the Couette CPMAto improve its transfer function and it is the key difference between itand the APM. However, as shown in the example above (FIG. 3B) the peakof the transfer function without the speed difference is already 0.95,therefore the added complexity of rotating the cylinders at slightlydifferent speeds is mostly likely not worth the slight improvement intransfer function. If different rotational speeds are used it should benoted that the speed ratio must satisfy the Rayleigh criterion (i.e., itmust not be the case that (r₁/r₂)²>ω₂/ω₁) beyond which the flow becomesunstable, thereby disturbing the classification of the particles.

Thirdly, the above description describes how the APC can be used toproduce a monodisperse aerosol based on the particle's aerodynamicdiameter, much like how a DMA is used to produce a quasi-monodisperseaerosol based on the particle's electrical mobility diameter. DMA's areoften combined with a condensation particle counter (CPC) to measure thenumber concentration of the quasi-monodisperse aerosol. Typically, thevoltage controlling the electrostatic field in the DMA is ‘scanned’ overthe range of the instrument, and by completing a data inversion of theCPC data, the size distribution of the aerosol can be determined. Thiscombination of DMA and CPC is normally called, a Scanning MobilityParticle Sizer (SMPS). The same method can be employed by combining theAPC with a CPC, or any other particle counting device, and bycontinuously ‘scanning’ the rotational speed or intermittently steppingthe rotational speed. That is, a particle counter could be placed in orconnected to the particle classification system or the outlet of theparticle classification system in order to detect the concentration ofparticles of an aerodynamic diameter or other derived metric allowingthem to reach the particle counter; the acceleration provided to theflow could be varied continuously or in steps, the aerodynamic diameter(or other metric related to the aerodynamic diameter) required to reachthe particle counter changing with the acceleration provided to theflow, so that the particle counter measures a spectrum of aerodynamicdiameter (or other metric related to the aerodynamic diameter) versusconcentration as the acceleration provided to the flow varies. Referringto FIG. 7A, an aerodynamic classifier 500, which may be of any of theembodiments described above, is shown with a particle counter 540. Aninitial aerosol flow 536 enters the classifier which acts on the aerosolflow to produce a classified flow 538 containing a selected portion ofthe aerosol particles present in the initial aerosol flow. Theclassified flow may be for example a sampling flow. The particle countermay be for example a condensation particle counter. The embodiment shownin FIG. 7A may include embodiments used to measure a spectrum ofaerodynamic diameter versus concentration or other embodiments.

Another way the APC can be used to measure aerosol size distributions isby eliminating the aerosol exit slit and placing in the flow channel oneor more detectors at which the particles may impact depending on theirtrajectory and measuring the number of particles impacting the one ormore detectors. In an embodiment a detector may comprise a conductorconnected to an electrometer circuit, for example in an annularembodiment, the detector may comprise a conducting ring connected to anelectrometer. The ring may be electrically isolated from both theremainder of the surface defining the flow channel and any otherdetection rings which may be present. The detectors may be situated atdifferent axial locations along the outermost surface of the flowchannel. Referring to FIG. 6, an APC 400 is shown having detectors 428located in the flow channel at which particles may impact depending ontheir trajectory. In the embodiment shown the flow channel is defined bycylinders as in FIG. 1, although other shapes would also work. In thisembodiment the detectors may be rings, electrically connected toelectrometer circuits, extending around the inside of the outercylinder. No sampling exit is necessary to classify particles whendetectors are used to detect impacting particles, although the detectorscould also be used in embodiments with a sampling exit. The embodimentshown in FIG. 6 may otherwise operate similarly to the embodiment ofFIG. 1, in particular, the rotating shaft 420, bearings 422, pulley 424,sheath flow 408, aerosol flow 406, initial flow channel 426, and exhaustflow 416 may be similar to their counterparts of FIG. 1 and cooperatesimilarly, except that the articles of the aerosol flow are charged. Inthis system, the particles to be measured would be charged (most likelywith a corona discharge-type charger or any other particle chargingmethod, see Hinds, Aerosol Technology, Wiley, 1999). The chargedparticles would move down the classification section and impact theelectrometer rings on the outer cylinder, thereby causing a measurablecurrent in the electrometer ring, where the current is proportional tothe number concentration of particles impacting the electrometer. Largerparticles would impact the electrometer rings near the aerosol entranceand smaller particles would impact the rings near the aerosol exit.Referring to FIG. 7B an APC 400 with electrometers, such as for examplethe APC shown in FIG. 6, is shown with a particle charger 430. Anuncharged aerosol flow 432 enters the particle charger 430 to produce acharged aerosol flow 434 comprising charged aerosol particles. The APC400 operates on the charged aerosol flow 434 to classify the chargedaerosol particles. By using a data inversion routine, the aerosol sizedistribution can be determined. Similar techniques have been used inDMA-like instruments like the differential mobility spectrometer(Reavell et al., A fast response particulate spectrometer for combustionaerosols. Society of Automotive Engineers, 2002) and the engine exhaustparticle sizer (Johnson et al., An engine exhaust particle sizerspectrometer for transient emission particle measurements. Society ofAutomotive Engineers, 2004); where electrometer rings have been placedinside a DMA-like classification column.

Thus the applicant has devised a new instrument, called the AerodynamicParticle Classifier (APC). As indicated, a detailed theoretical modelhas been developed for the instrument. The model shows the instrumentcan have excellent classification properties (i.e. wide range, highresolution, and high penetration efficiency) without requiring particlecharging. This results in an instrument that in an embodiment canproduce a true monodisperse aerosol without classifying multiply-chargedparticles like the DMA, APM, or CPMA. An APC could be combined in serieswith a DMA or CPMA in order to measure other important particleproperties including: mobility diameter, particle mass, effectivedensity, fractal-like dimension, and dynamic shape factor.

Immaterial modifications may be made to the embodiments described herewithout departing from what is covered by the claims.

In the claims, the word “comprising” is used in its inclusive sense anddoes not exclude other elements being present. The indefinite article“a” before a claim feature does not exclude more than one of the featurebeing present. Each one of the individual features described here may beused in one or more embodiments and is not, by virtue only of beingdescribed here, to be construed as essential to all embodiments asdefined by the claims.

1. A method of classification of particles suspended in a fluidcomprising the steps of: providing a carrier flow of a fluid; supplyingparticles into suspension in the carrier flow; providing an accelerationto the flow at an angle to the velocity of the flow to cause theparticles to follow trajectories determined by the acceleration and dragon the particles caused by the fluid; and classifying the particlesaccording to the trajectories of the particles.
 2. The method of claim 1in which the fluid is a gas and the particles are aerosol particles. 3.The method of claim 2 in which the fluid is air.
 4. The method of claim1 in which the velocity of the fluid is tangential to a cylinderdefining a flow channel of the carrier flow of the fluid.
 5. The methodof claim 1 in which the acceleration is centripetal acceleration.
 6. Themethod of claim 1 in which the step of supplying particles intosuspension in the carrier flow comprises merging a flow of a fluidcontaining suspended particles into the carrier flow.
 7. The method ofclaim 1 in which the step of classifying particles according to thetrajectory of the particles comprises splitting the carrier flow intotwo or more flows.
 8. The method of claim 1 in which the step ofclassifying the particles according to the trajectory of the particlescomprises supplying a surface at which particles may impact depending ontheir trajectory.
 9. The method of claim 7 further comprising the stepsof varying the acceleration provided to the flow and measuring theconcentration of particles in at least one of the two or more flows asthe acceleration provided to the flow is varied.
 10. The method of claim8 further comprising the steps of varying the acceleration provided tothe flow and measuring the concentration of particles in a continuingflow in a downstream direction from the surface as the accelerationprovided to the flow is varied
 11. The method of claim 9 in which themeasurement of concentration at varying acceleration is used to obtain aspectrum of concentration spectral density versus aerodynamic particlediameter or particle mass to drag ratio or equivalent metric.
 12. Themethod of claim 8 in which the step of classifying the particlescomprises supplying one or more detectors at which the particles mayimpact depending on their trajectory and measuring the number ofparticles impacting the one or more detectors.
 13. The method of claim12 further comprising the step of charging the particles and in whichthe detectors are electrically conductive and are each connected to anelectrometer circuit.
 14. The method of claim 1 in which the carrierflow is provided through a channel having an inner rotational surfaceand the particles are supplied into the carrier flow through the innerrotational surface.
 15. The method of claim 1 in which the particles areclassified within a range of particle sizes between a maximum size and aminimum size.
 16. An apparatus for classifying particles suspended in afluid, the apparatus comprising: one or more elements defining a carrierflow channel or plurality of channels; a source of a carrier fluid flowinto the carrier flow channel or channels; a source of particlesconnected to supply the particles into suspension in the carrier fluidin the carrier flow channel or channels; a drive connected to operate onthe elements defining the carrier flow channel or channels to supply anacceleration to the elements defining the flow channel or channels at anangle to the flow of fluid through the carrier flow channel or channels;and a classification system for classifying the suspended particlesaccording to their trajectories.
 17. The apparatus of claim 16 in whichthe carrier fluid is a gas and the particles are aerosol particles. 18.The apparatus of claim 17 in which the carrier fluid is air.
 19. Theapparatus of claim 16 in which the one or more elements defining acarrier flow channel substantially form surfaces of rotation about acentral rotational axis.
 20. The apparatus of claim 17 in which thesource of particles is supplied into the carrier flow channel by anopening substantially in the inner rotational surface of the carrierflow channel.
 21. The apparatus of claim 16 in which the carrier flowchannel is substantially cylindrical in shape.
 22. The apparatus ofclaim 21 in which the elements defining the carrier flow channel arecylinders.
 23. The apparatus of claim 16 in which one or more of theelements defining the carrier flow channel or channels substantiallyform sectors of surfaces of rotation about a central rotational axis.24. The apparatus of claim 23 in which one or more of the elementsdefining the flow channel or flow channels are substantially shaped assectors of a cylinder.
 25. The apparatus of claim 16 in which the drivecomprises a motor connected to cause rotation of the elements definingthe carrier flow channel or channels.
 26. The apparatus of claim 16 inwhich the source of particles connected to supply the particles intosuspension in the carrier fluid in the carrier flow channel or channelscomprises elements defining a suspension flow channel or channels whichintersects the carrier flow channel or channels, the suspension flowchannel or channels being capable of directing a fluid containingsuspended particles into the carrier flow channel or channels.
 27. Theapparatus of claim 16 in which the classification system compriseselements defining a split of the carrier flow channel or channels intotwo or more channels.
 28. The apparatus of claim 16 in which theclassification system comprises a surface of each carrier flow channelat which particles suspended in fluid in the carrier flow channel mayimpact depending on their trajectory.
 29. The apparatus of claim 28 inwhich the surface is an element defining or partially defining thecarrier flow channel.
 30. The apparatus of claim 16 in which theclassification system comprises an outlet connected to or incorporatinga particle detector, the drive having a variable rotation speed.
 31. Theapparatus of claim 16 in which the source of particles comprises acharging means for charging the particles and the classification systemcomprises at least a conductive detector placed along the flow channel,the at least a conductive detector connected to an electrometer circuit.32. The apparatus of claim 16 in which the classification systemoperates by classifying particles within a range of particle sizesbetween a maximum size and a minimum size.